The present invention relates to a flat cable.
As compared with an electric wire having a round section, a flexible flat cable (hereinafter it may be abbreviated to FFC) refers to an electric wire composed of a plurality of conductors disposed parallel in the longitudinal direction, which are insulated and coated in an electrically independent state. The FFC is characterized by excellent flexibility, capability for being bent, and easy formation of multiple cores. Further, since the FFC is thin and can minimize the space occupied by the wire itself, it attracts attention as a wiring material to be used in automobiles and appliances, which wiring is advanced in the multi-core structure due to the recent complication in control.
The FFC using flat conductors having a rectangular sectional shape, in particular, is characterized by being high in heat dissipation, if the conductor sectional area is the same, because the surface area of the flat conductor is wider than that of a round-section conductor. In the case of multi-core FFC, the smaller the thickness of the flat conductor as compared with its width is, the better and more remarkable the heat dissipation property is.
Generally, the allowable current of an electric wire is determined by the degree of temperature rise when power is fed, the ambient temperature, and the heat resistance temperature of the coating material. The allowable current value of an electric wire is judged to be larger when the degree of temperature rise when power is fed is smaller, the ambient temperature is lower, and/or the heat resistance temperature of the coating material is higher.
Usually, an electric wire is protected by actuation of a fuse or circuit breaker to shut down the current if a large current flows, so a large current will not flow for a long period of time. Therefore, the heat resistance required in such a coating material means heat resistance in a very short period of time (about 500 to 1000 seconds), and it does not mean a long-term characteristic, such as resistance to aging by heat.
If heat resistance in a short period of time of the coating material can be enhanced, a large current can flow, which contributes greatly to saving of conductors to be used in wiring and reduction of weight.
Comparing the FFC using flat conductors and an ordinary round-section electric wire using round-section conductors, if the coating materials are the same and the sectional areas of conductors are the same, the flat conductor FFC can release the heat generated by power feeding more smoothly, and the degree of temperature rise of conductors is smaller, so that a larger allowable current may be determined. In other words, if the allowable current values are the same, the amount of conductors to be used can be lessened in the FFC than in the round-section electric wire, and the weight of the wiring material within the appliance can be reduced advantageously.
As the FFC producing method, the laminate method and extrusion molding method are representative. In the laminate method, conductors are held with films of insulators from both above and beneath, and passed through heated rolls, and the upper and lower films and conductors are integrated by adhering or fusing. In ordinary manufacturing, use is made of an adhesive layer that is applied on the surface of a very thin film made of polyethylene terephthalate or the like, or use is made of these layer and film that are co-extruded.
In the extrusion molding method, for example, a polyamide, a polyolefin, or a polyamide polymer alloy is used as a coating material.
The laminate-type FFC is accompanied with such problem that the speed of manufacturing line is very slow, and that it is hence low in productivity. This is because heat must be transmitted from the heated roll to the adhesive layer held between films, by way of the films, which are insulating and very poor in heat transfer, and it takes time to transfer enough heat to the adhesive layer. The adhesive layer is required to be fused easily by heat. Therefore, it is difficult to expect sufficient heat resistance in the FFC produced by such a laminate method using heat.
On the other hand, FFC made by the extrusion molding method is advantageous as compared with that of the laminate type in heat resistance, because it has no adhesive layer. Extrusion coating materials include polyvinyl chloride (hereinafter it may be abbreviated to PVC), polyamide, and polyolefin, as typical examples.
Polyamide is generally very high in moisture absorption, and it is accompanied with the problem of significant change in mechanical properties and electrical properties between the dry state and the moist state. An example of a polyamide relatively low in moisture absorption, and evaluated as an electric wire coating material, is nylon 12. However, nylon 12 has a low melting point, and it fuses at about 180xc2x0 C. On the other hand, PVC derivative coating materials used hitherto hardly deteriorate at this level of temperature, if exposed for a short period of time, and they can maintain performance. Therefore, if nylon 12 is used as the coating material for FFC, a saving effect of conductors to be used is hardly obtained as compared with PVC, from the viewpoint of improving the heat resistance temperature.
On the other hand, polyolefin is low in moisture absorption, and it is inexpensive and excellent in extrusion molding performance, but it is low in heat resistance; and, for example, polypropylene begins to soften at about 150xc2x0 C. Therefore, polyolefin also does not contribute to saving of conductors to be used, from the viewpoint of improving the heat resistance temperature of the coating material.
Further, the PVC has a high content of halogen-group chlorine, and it is high in environmental load that is the problem.
Accordingly, to minimize the sectional area of conductors without lowering the allowable current value, it is required to enhance the heat resistance of the coating material sufficiently. A thermoplastic aromatic polyester is known as a relatively inexpensive material having a high heat resistance temperature. Representative examples of ordinary thermoplastic aromatic polyester to be used include polyethylene terephthalate (hereinafter abbreviated to PET) and polytetramethylene terephthalate (hereinafter abbreviated to PBT). The PET film used in laminate-type FFC has already been drawn in the film extrusion molding process; it has a high degree of crystallization; it is very uniform and homogeneous in the crystal size, and it is tough and high in the level of resistance to the hydrolysis that is evaluated in the present invention. However, in the case of the extrusion molding method, since there is no drawing process, the degree of crystallization is insufficient in both PET and PBT, and the crystal size is not uniform, and hence it is brittle and inferior in flexibility, and is extremely lowered in resistance to the hydrolysis that is evaluated in the present invention. Therefore, thermoplastic aromatic polyester resin has been hardly used so far as a coating material preferable for extrusion molding of FFC.
The present invention is a flat cable that has an insulating coating composed of a polyester resin composition, which composition comprises:
(A) 100 parts by weight of a thermoplastic aromatic polyester;
(B) 10 to 120 parts by weight of a polyester block copolymer composed of: 20 to 80 wt % of a hard segment whose main component is poly(tetramethylene terephthalate) that is composed of an acid component in which terephthalic acid occupies 60 mole % or more and a diol component in which tetramethylene glycol occupies 60 mole % or more; and 80 to 20 wt % of a soft segment whose main component is a polyester that is composed of an acid component in which an aromatic dicarboxylic acid occupies 60 mole % or more and a diol component of a long-chain diol having 5 to 12 carbon atoms; and
(C) 1 to 50 parts by weight of an olefin-acrylate copolymer modified by a glycidyl compound.